Most of the training equipment present on the market today are designed according to a few construction concepts: devices based on the movement of weights, devices comprising springs and other elastic elements, devices based on friction, actuators like clutches, brakes, fluid valves, (pneumatic, hydraulic), etc. and motor-driven devices.
In order to gain an insight into a training progression and to optimise the training result, it is extremely important to control the relevant movement parameters for muscles such as: load force, contraction speed, acceleration etc. The essential accent in this direction is to be able to exercise muscles with given load values.
When using weights, the gravitation force is used in order to obtain a load on the muscles. The mass of the weights is given and corresponds to the force of the weights during rest only. When lifting the weights during a certain time interval its mass is accelerated unavoidably. Any acceleration of a mass creates time dependent forces of inertia that are the product of the mass and the acceleration values during that time period.
From the medical, exercising and competition experience it is widely known that load variations caused by inertial force can be significant. Therefore, in order to enable some reasonably acceptable controlled training and avoid muscle and ligament injuries, lifting of weights has to be performed with as low as possible acceleration. Due to a relatively short weight lifting length, only relatively low speeds can be used in order to have a low acceleration. It will therefore be impossible during training with weights, or weight-based training equipment, to perform a movement with both arbitrary given muscle contraction loads and speeds simultaneously. Inertial force drastically restricts the freedom regarding selection of speed and acceleration in exercise. The limitation lies in the fact that instantaneous muscle power, strength or effects (product of muscle force and contraction speed) appearing during acceleration of a weight, can easily exceed a maximal tolerable value of a muscle, which value the muscle can't reach, or if reached the muscle can be injured. Consequently it is practically impossible to regularly exercise of the essential physical training magnitude i.e. the actual muscle strength.
During training with a so-called “isokinetic” machine, the problem is the reverse. In this case the speed of the muscle contraction is given, while the muscle load is arbitrarily fluctuating.
Further, weight-based training equipment has other drawbacks depending on their weight. They must therefore be placed in training facilities with robust under-carriage and should not be in movement or be swinging. Because weights during lifting can be moved only vertically, a certain orientation in space is always needed, which limits the freedom of the construction and the installation possibilities.
With friction-based equipment, a load is obtained which is dependent partly on acceleration, but particularly on speed. By continuously controlling a friction force with breaks, clutches and valves, the dependency of the movement dynamics can partly be reduced. However, the major drawback with using friction forces is that they are reactive and thereby passive, which prevents training with very favourable and desirable so called negative muscle work.